The present invention relates to a method and an apparatus for determining a plurality of spatial coordinates on a measurement object having a plurality of measurement features. Such a method is often carried out using what is known as a coordinate-measuring machine and, accordingly, the present invention particularly relates to a method and an apparatus involving a coordinate-measuring machine. However, the invention is not limited to conventional coordinate-measuring machines. It can likewise advantageously be implemented using one or more robots or any other machine structures allowing to displace a measurement object and a working head relative to each other.
WO 2014/023332 discloses a coordinate-measuring machine having a measurement head which carries an optical sensor. The measurement head is displaceable relative to a workpiece table such that it can be moved into various measurement positions relative to a measurement object. The respective working position of the measurement head relative to the workpiece table is determined by way of encoder arrangements which have linear scales along the movement axes of the measurement head or of the workpiece table.
The accuracy with which the measurement head can be positioned relative to the measurement object on the workpiece table influences the measurement accuracy with which the measurement object can be measured. For this reason, highest possible positioning accuracy is desirable. The same is true for the working head of a machine tool or a robot for processing an object. Generally, the positioning accuracy of a machine working head depends on many factors, especially on the accuracy and the stability of the mechanical guides, on the accuracy of the linear scales and on the accuracy of the encoder arrangement with which the position of the working head within the working space is typically determined. realization of such components becomes more complex with rising accuracy requirements.
In order to achieve high positioning accuracy of a working head, typically a heavy machine construction is required. This frequently runs counter to the desire to realize high accelerations of the working head for a fast performance of the working process. For this reason, the engineer for an apparatus having a moving working head must frequently strike a compromise which limits the maximum positioning accuracy that is possible. Furthermore, ageing, wear, temperature variations and other environmental conditions may cause the positioning accuracy of a working head to be limited and/or to decrease over time.
It is a known procedure to determine positioning errors of a coordinate-measuring machine during what is referred to as a calibration for selected working positions and to correct the corresponding positioning errors during the working process by way of calculation. However, any correction of this kind means extra outlay and can be optimum merely for selected working positions.
EP 2 486 369 B1 discloses a coordinate-measuring machine having a tactile sensor which can be moved along three mutually orthogonal coordinate axes. Again, the respective working position of the tactile sensor is determined using linear scales. Moreover, the described coordinate-measuring machine has a plurality of change-in-location sensors which provide acceleration data for the tactile sensor. According to an exemplary embodiment, the data of the linear scales can be reconciled with the data of the change-in-location sensors. This permits plausibilization and furthermore additionally a correction of position information which are determined using the linear scales. However, the known coordinate-measuring machine requires the additional change-in-location sensors, which constitutes a disadvantage for some applications.